Auditory System Flashcards
What is sensing sound important for? (5)
- Communication
- Triggers emotion
- Recognising what things are by their sound
- 3D view of the world
- Survival, escaping danger
What are the units of frequency?
Hertz (Hz)
What are the units of loudness (intensity)?
Decibels (dB)
What are the 3 sections of the ear?
- Outer ear
- Middle ear
- Inner ear
How does sound travel across the middle ear?
Via the ossicles
What are the 3 chambers of the cochlea?
- Scala vestibuli
- Scala media
- Scala tympani
What is the organ of Corti?
The sensory organ of the cochlea containing the hair cells
What is the innervation of the organ of Corti?
Auditory nerve fibres
Where is the organ of Corti located?
Sits on the basilar membrane inside the scala media
What are the features of the organ of Corti? (3)
- Contains 3 rows of outer hair cells and 1 row of inner hair cells
- Sits on the basilar membrane
- Tectorial membrane sits over the top of the hair cells
What is the arrangement of the 3 chambers in the cochlear spiral?
- Scala media in the middle
- Scala vestibuli above
- Scala tympani below
What is the composition of perilymph? (4)
- Normal extracellular solution
- Low K+
- High Na+
- Normal Ca2+
Which chambers of the cochlea contain perilymph?
- Scala vestibuli
- Scala tympani
What is the composition of endolymph? (3)
- High K+
- Low Ca2+
- Low Na+
Which chambers of the cochlea contain endolymph?
Only the scala media
How is the high K+ content of the endolymph generated?
Cells in the stria vascularis actively pump K+ into the scala media
What is the endocochlear potential?
Positive potential of +80mV in the scala media compared to the other chambers
What is the resting membrane potential of the hair cells in the organ of Corti?
-60mV
What are the 2 types of hair cells?
Outer and inner hair cells
Why is the cochlea arranged in a spiral structure?
- To extend the frequency range of hearing as much as possible
- Fits more hair cells within the small space
What is the tonotopic organisation of the cochlea? (2)
- Cells at the base respond to high frequency sounds, cells at the apex respond to low frequency sounds
- This map is preserved throughout the auditory pathway into the brain
How is sound frequency encoded? (3)
- Place-Frequency code
- The brain interprets the frequency of a sound by the position of the hair cells that are activated
- Frequency isn’t encoded in the firing pattern of the nerves as it is represented by location
What determines the cochlear tonotopicity?
The basilar membrane travelling wave which goes from base to apex
What is the Characteristic Frequency (CF) location?
The location where the same sound frequency causes maximal movement of the basilar membrane
Which part of the basilar membrane is maximally displaced by low frequency sound?
Apex
Which part of the basilar membrane is maximally displaced by high frequency sound?
Base
Which part of the basilar membrane is narrow and stiff?
Base
Which part of the basilar membrane is wide and floppy?
Apex
What determines the CF location of a sound?
Width and stiffness of the basilar membrane
Why do high frequency sounds have a CF location at the base of the basilar membrane?
Short wavelength and low energy so don’t travel very far
Why do low frequency sounds have a CF location at the apex of the basilar membrane?
Long wavelength and high energy so travel further
What is the function of the inner hair cells? (2)
- They are the primary sensory receptors of the auditory system
- They encode all auditory information and pass it on to the nerve fibres
What are the features of inner hair cells? (3)
- Hair bundle of stereocilia on the apical surface
- The shorter stereocilia have MET channels at their tips
- MET channels on shorter stereocilia are connected to the taller ones by tip links which pull the channels open
What are MET channels?
Mechanoelectrical Transducer Channels
What ion channels do inner hair cells express? (2)
- Voltage gated Ca2+ channels to trigger neurotransmitter release
- Voltage gated K+ channels for repolarisation
Why are inner hair cells connected to many afferent nerve fibres?
They are the main sensory receptors of the auditory system
What happens in the inner hair cells at rest (no sound)? (4)
- Slight tension on the tip links which opens some MET channels resulting in a resting current (ITRest)
- K+ enters the cell from the endolymph down a large electrical gradient
- K+ leaves the cell via K+ channels down a large concentration gradient into the surrounding perilymph
- Some Ca2+ and therefore neurotransmitter is released, causing some resting activity in the afferent fibres (spontaneous rate)
What kind of gradient allows K+ to enter the inner hair cells from the endolymph? (2)
- Electrical gradient (endolymph is +80mV, hair cell resting potential is -60mV)
- Endolymph and intracellular K+ is similar so not much of a concentration gradient
What is the spontaneous rate?
The firing of the afferent nerve fibres in the absence of any sound stimulus
What happens in the inner hair cells during excitatory stimulation? (5)
- Shorter stereocilia are pulled towards the taller ones which pulls the tip links and opens the MET channels
- Large MET current depolarises the hair cell
- Ca2+ channels open and neurotransmitter is released
- Increases the firing activity of the afferent nerve fibres above the resting level
- Voltage gated K+ channels open and K+ exits down a concentration gradient into the surrounding perilymph causing repolarisation
What happens in the inner hair cells during inhibitory stimulation? (5)
- Taller stereocilia are pushed towards the shorter ones which reduces tension in the tip links and closes the MET channels
- The cell hyperpolarises below its resting level
- Little to no Ca2+ and neurotransmitter release
- Firing of the afferent nerve fibres drops below the spontaneous rate
- K+ channels remain open for longer to repolarise the cell
What happens to an inner hair cell during a sustained sound? (3)
- Hair bundle is moved back and forth
- Creates a cycle of depolarisation and hyperpolarisation which matches the sound frequency
- Generates pulses of neurotransmitter release and afferent activity
What is the advantage of using K+ to both depolarise and repolarise the cell?
Rapid and energy efficient (no active transport needed to maintain gradients)
What kind of gradient allows K+ to leave the inner hair cells into the perilymph?
Chemical gradient (high intracellular K+ and low K+ in the perilymph)
What happens if the barrier between the endolymph and perilymph breaks down?
Causes deafness
What is the function of outer hair cells?
Cochlear amplification
What makes the outer hair cells electromotile?
They shorten and lengthen in response to sound
Why do outer hair cells not have many afferent nerve fibres? (2)
- They don’t have a main sensory role
- Most of their innervation is from inhibitory efferent fibres from the brain
What are the features of outer hair cells? (2)
- V shaped hair bundle on the apical surface
- Stereocilia work in the same way as the inner hair cells
What ion channels are expressed by outer hair cells?
Mainly voltage gated K+ channels
What molecule present in the cell membrane of outer hair cells allows them to change length?
Prestin
What is the resting potential of outer hair cells?
-40mV
What happens to outer hair cells during excitatory stimulation? (3)
- Hair bundle is deflected towards the taller stereocilia
- Cell depolarises
- Shortens in length
What happens to outer hair cells during inhbitory stimulation? (3)
- Hair bundle is deflected towards the shorter stereocilia
- Cell hyperpolarises below the resting potential
- Gets longer
How do the outer hair cells amplify sound? (2)
- They act as positive feedback in the cochlea by increasing the movement of the basilar membrane over a narrow CF region
- This increases the stimulation of the inner hair cells and increases their sensitivity
What happens if you lose your outer hair cells?
Severe hearing loss but not deafness because you still have the inner hair cells and the basic tuning of the basilar membrane
